Cellular Structures and Processes: Study Notes for Anatomy & Physiology

Cellular Transport Mechanisms

Plasma Membrane Pumps

Plasma membrane pumps are specialized proteins that actively transport substances across the cell membrane, often against their concentration gradients.

• Example 1: Sodium-Potassium Pump (Na+/K+ ATPase): Maintains cellular ion balance by pumping 3 Na+ ions out and 2 K+ ions into the cell. Energy Source: ATP hydrolysis.

• Example 2: Calcium Pump (Ca2+ ATPase): Removes Ca2+ from the cytoplasm to the extracellular space or into organelles. Energy Source: ATP hydrolysis.

• Outcome: Maintains electrochemical gradients essential for nerve impulse transmission and muscle contraction.

Active Transport: Primary vs. Secondary

Active transport moves substances against their concentration gradients using energy.

• Primary Active Transport: Direct use of ATP to transport molecules (e.g., Na+/K+ pump).

• Secondary Active Transport: Uses energy stored in the form of an ion gradient created by primary active transport (e.g., glucose transport via Na+ gradient).

Symporters vs. Antiporters

These are types of membrane transport proteins involved in secondary active transport.

• Symporter: Moves two substances in the same direction across the membrane (e.g., Na+-glucose symporter).

• Antiporter: Moves two substances in opposite directions (e.g., Na+/Ca2+ exchanger).

Endocytosis and Exocytosis

Definitions and Differences

Endocytosis and exocytosis are processes for bulk transport of materials across the cell membrane.

• Endocytosis: The process by which cells take in substances by engulfing them in a vesicle.

• Exocytosis: The process by which cells expel substances by fusing vesicles with the plasma membrane.

• Difference: Endocytosis brings materials into the cell; exocytosis releases materials out of the cell.

Types of Endocytosis

• Phagocytosis: "Cell eating"; uptake of large particles or cells.

• Pinocytosis: "Cell drinking"; uptake of fluids and dissolved substances.

• Receptor-Mediated Endocytosis: Specific uptake of molecules after binding to cell surface receptors.

Cell Structures and Their Functions

Major Cell Structures

Cells contain various organelles, each with specialized functions.

Structure	Function
Nucleus	Contains genetic material (DNA); controls cell activities.
Endoplasmic Reticulum (ER)	Protein and lipid synthesis; rough ER has ribosomes, smooth ER does not.
Golgi Apparatus	Modifies, sorts, and packages proteins and lipids for secretion or delivery.
Lysosomes	Digestive organelles; break down waste and cellular debris.
Peroxisomes	Break down fatty acids and detoxify harmful substances.
Mitochondria	Produce ATP via cellular respiration.
Cytosol	Fluid portion of cytoplasm; site of many metabolic reactions.
Vesicles	Transport materials within the cell.
Ribosomes	Sites of protein synthesis.

DNA Structure and Function

Nucleus and DNA

The nucleus is the control center of the cell, housing DNA in the form of chromatin or chromosomes.

• Structure of the Nucleus: Surrounded by a double membrane (nuclear envelope); contains nucleolus and chromatin.

• Structure of DNA: Double helix composed of nucleotides (adenine, thymine, cytosine, guanine).

• Basic Roles of DNA: Stores genetic information; directs synthesis of proteins.

Gene Expression: Transcription and Translation

Definitions and Differences

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of protein from an mRNA template.

• Difference: Transcription produces RNA; translation produces protein.

Phases of Transcription and Translation

• Transcription Phases: Initiation, elongation, termination.

• Translation Phases: Initiation, elongation, termination.

Types of RNA and Their Functions

Type of RNA	Function
mRNA (messenger RNA)	Carries genetic code from DNA to ribosome.
tRNA (transfer RNA)	Brings amino acids to ribosome during protein synthesis.
rRNA (ribosomal RNA)	Forms the core of ribosome's structure and catalyzes protein synthesis.

Promoter, Exon, and Intron Regions

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Exon: Coding region of a gene; expressed in the final mRNA.

• Intron: Non-coding region; removed during RNA processing.

DNA Replication

Process and Enzymes

DNA replication is the process by which a cell duplicates its DNA before cell division.

• Role of DNA Polymerase: Synthesizes new DNA strands by adding nucleotides to a template strand.

• Key Steps: Unwinding of DNA, complementary base pairing, joining of nucleotides.

Formula:

$\text{DNA Polymerase:} \quad \text{(DNA template)} + \text{dNTPs} \rightarrow \text{New DNA strand}$

Cell Cycle and Mitosis

Phases of the Somatic Cell Cycle

The somatic cell cycle consists of stages that prepare a cell for division.

• G1 Phase: Cell growth and preparation for DNA replication.

• S Phase: DNA synthesis (replication).

• G2 Phase: Further growth and preparation for mitosis.

• M Phase: Mitosis and cytokinesis (cell division).

Mitosis and Somatic Cell Division

Mitosis is the process by which a cell divides to produce two genetically identical daughter cells.

• Phases of Mitosis: Prophase, metaphase, anaphase, telophase.

• Role: Promotes growth, tissue repair, and asexual reproduction in somatic cells.

Additional info: Ribosomes coordinate translation by reading mRNA codons and facilitating the assembly of amino acids into polypeptides.